The present invention relates to a supercharging pressure control system for an internal combustion engine with a turbocharger, and more particularly to a supercharging pressure control system for accurately controlling intake passage pressure downstream of a throttle valve to a target value by controlling supercharging pressure in dependence on operating conditions of the engine while the throttle valve is opened to a wide extent.
Conventional internal combustion engines include a type including a turbocharger which is controlled by a waste gate valve. An internal combustion engine of this type is adapted such that an increased quantity of intake air is introduced into the engine cylinders by maintaining intake air pressure in the intake passage at the downstream side of a compressor of the turbocharger higher than atmospheric pressure to increase the charging efficiency of the engine and thereby increase the engine output. Further, the pressure in the intake passage is controlled by actuating the waste gate valve to selectively close and open a bypass passage or exhaust branch passage in response to the same pressure in the intake passage, thereby adjusting the flow rate of exhaust gases to be introduced into the turbine and thus the rotational speed of the turbine, that is, the compressor.
In an internal combustion engine of the type including a throttle valve disposed in an intake passage thereof at the downstream side of the compressor, the charging efficiency of the engine depends upon pressure in the intake passage at the downstream side of the throttle valve (hereinafter referred to as the P.sub.B pressure), as is well known. Therefore, it is desirable in such an engine to control the P.sub.B pressure to a desired value so as to achieve an optimum value of charging efficiency. Further, the P.sub.B pressure forms, together with intake air temperature, main factors affecting the occurrence of knocking of the engine. In order to avoid knocking of the engine as well as to achieve optimum engine output, it is necessary to regulate pressure in the intake passage at a zone downstream of the compressor and upstream of the throttle valve, that is, supercharging pressure, so as to obtain the above desired value of P.sub.B pressure, by means of the waste gate valve. Usually, the P.sub.B pressure is not greatly affected by the supercharging pressure as the throttle valve is increasingly throttled, whereas the P.sub.B pressure approaches the supercharging pressure as the opening of the throttle valve increases. By this reason, it is effective to control the supercharging pressure so as to obtain a required P.sub.B pressure value particularly when the throttle valve is opened to a wide extent.
On the other hand, a conventional supercharging pressure control system for an internal combustion engine with a turbocharger is constructed such that the supercharging pressure at the upstream side of the throttle valve is controlled by an actuator of the pneumatic type for actuating the waste gate valve simply in response to the same supercharging pressure, or such that a control valve is provided for controlling communication of the pressure chamber in such pneumatic type actuator with the atmosphere and controlled by means of other parameters than the P.sub.B pressure. Thus, the conventional supercharging control system does not employ the P.sub.B pressure as a primary control parameter, and therefore is unable to achieve optimum values of charging efficiency of the engine or output thereof properly corresponding to operating conditions of the engine.